Research and development of a component technology for implementing various ubiquitous sensor networks are being actively carried out. As subjects for widely spreading this technology, downsizing of sensor terminals for not making aware of locations and possession thereof, yearly power consumption reduction, life-lengthening and real time response performance of the battery life are sought for (Non Patent Document 1).
In communication technologies such as ZigBee, Bluetooth (registered trademark), UWB and the like research and development of which are being promoted by conventional sensor network terminals, the power consumption reduction by a sleep periodic startup operation sacrificing the real time response performance is promoted with competition for the battery life. Although application of such sensors to a system that sufficient effect can be expected if communication is established at a rate of one time in every five minutes as in sensing or the like of climatic environment is effective, realization of real time bidirectionality becomes an important subject in systems for sensing/guiding risk information and supporting/watching aged persons and others which are listed in the future vision (Non Patent Document 1) of the ubiquitous sensor networks.
The inventors and others have promoted development of small-sized and low power consumption wireless connection sensor terminals and sensor network systems using the ever proposed wearable antenna (Patent Document 1), and the low power consumption and real time bidirectional wireless connection technology (Patent Document 2) for the purpose of contributing to construction of the ICT reliable and safe social infrastructure. In addition, they experimentally manufacture and evaluate sensor tags which operate under the ARIB STD-T81 standard and a reading operation up to 35 m of which has been confirmed with 2.45 GHz band 3 mW/MHz reader output. These tags are three kinds of sensor wireless tags for 20-time per second continuous ranging, tri-axial acceleration sensor wireless tags for operating in 350-time per second continuous sampling and electrocardiograph sensor wireless tags, and any of them is about 3 mm in thickness, is soft and can be integrated with a dress. In addition, the consumption current of a built-in 3V coin-cell battery in continuous operation was 10 μA, 633 μA and 583 μA in the respective tags (Non Patent Document 2).
In the present specification, there will be disclosed a small-sized, low power consumption and long-life real time wireless data communication terminal which allows positioning in the 5 GHz band for a wireless distributed sensor network targeting on a moving body such as ITS or the like which is being developed aiming at power consumptions of not more than 500 μW in continuous transmission and reception of 100 kbps data and of not more than 10 μW in standby, and a communicable range of at least 30 m with an positioning error of not more than 10 cm.
First, the basic study to attain range lengthening and power consumption reduction of a 5 GHz band sensor tag will be described and then a flexible cavity-backed slot antenna, a stub resonance booster receiving circuit, and a pulse coding key detection and subcarrier MPSK modulation system for a waking-up operation which is reduced in power consumption and malfunction will be disclosed as techniques for range lengthening and power consumption reduction.
In addition, a DSP receiver for reader which has been experimentally manufactured for high speed/high precision positioning, and performs inverse Fourier transformation and interpolation ranging process of frequency response of a reflected subcarrier signal in the 200 MHz band width within 5 ms will be also disclosed.
FIG. 1 is an example of a conventional passive wireless tag device and a circuit diagram described in FIG. 2 of Patent Document 3.
In this figure a capacitor C1 connected to an antenna element L2 resonates with a λg/4 short stub L3 to boost the amplitude of an antenna received RF signal by about 10 times. D3 rectifies and detects it to charge C3, and the RF signal which has been boosted by the stub voltage-doubles and rectifies by being connected to D4 and D5 via C2. Since a reference potential of the above-mentioned voltage doubler rectifier circuit works as a charge voltage of C3, a tripled rectified voltage of the above-mentioned stub boosted RF signal is charged to C4.
In this circuit, a 30-time multiplied rectified voltage of the received RF signal which has been charged to C4 is used for driving a microprocessor U7 or the like, and a 10-time multiplied rectified voltage of the received RF signal which has been charged to C3 is used for ASK demodulation of the received signal. Here, it is described that the reason for proper use of two kinds of rectified voltages of the stub boosted RF signal lies in that a comparatively high drive voltage is needed for the above-mentioned U7 and the like, and a short time constant (a comparatively low output impedance) is required for ASK demodulation of the above-mentioned received signal.
However, all the embodiments described in Patent Document 3 including FIG. 1 are for the 2.45 GHz band (the UHF band), and it was found that a boosted rectified output voltage required for the wireless tag cannot be obtained in the microwave band (3-30 GHz). For example, the voltage to be charged to C3 in FIG. 1 in the 5 GHz band was only up to about 3 times the received RF signal voltage, and the voltage to be charged to C4 was only up to about 6 times the received RF signal voltage.
It is thought that the cause for large drop of the boost ratio lies in (1) the Q value of the short stub itself has been lowered and (2) the junction capacitance of D3 and the junction capacitances of D4 and D5 which are connected via C2 have worked as loads on the stub resonance booster circuit (the resonance circuit by C1 and L3) and have further lowered the Q value of the resonance circuit. Here, it is thought that (1) is caused by an increase in electromagnetic wave radiation loss from the stub short end with frequency rising and the cause of (2) lies in that a +j impedance component in stub resonance could not thoroughly compensate for a −j impedance component of the capacitance load due to (1).
FIG. 2 is a microwave band booster rectifier circuit of the present invention which is mainly used in a passive type wireless tag device. (a) and (b) of FIG. 2 are examples that a voltage-doubler rectifier circuit is combined with a λg/2 open stub resonant booster circuit, and (c) is an example that a voltage-quadrupler rectifier circuit and an ASK demodulation circuit are combined with the λg/2 open stub resonant booster circuit.
In case of the 5 GHz band in FIG. 2, C1=0.1 pF, C2-C4=1 pF, C5=L1-L3=5 nH, R1=10 kΩ, R2=2.2 MΩ, RL is an equivalent load resistance of the wireless tag circuit, D1 to D4 are DMF2828 (a Schottky barrier diode of Cj=0.1 pF), D5 is a backflow prevention diode, D6 is a Zener diode for protection, and U1 is a Schmitt trigger inverter. In a result of experiments on these circuits, with a resonance frequency set in the 5 GHz band, a −20 dBm sine wave input into RFin at 50Ω and RL=10 MΩ, V0=470 mV (21 times in boost ratio) in the case of (a), V0=420 mV (19 times in boost ratio) in the case of (b) and V0=720 mV (33 times in boost ratio) in the case of (c) were obtained. It can be said that the output voltage of (c) is a voltage sufficient to operate the wireless tag circuit. In addition, also in the configurations of (a) and (b), if about −15 dBm is present as the input power, it will be possible to operate the wireless tag circuit.
In the following, differences between FIG. 1 of the conventional system and FIG. 2 according to an embodiment of the present invention will be described.
A) In FIG. 1, the short stub is used for resonant boosting of the RF signal, and in FIG. 2, the open stub is used for it.
B) The reason for use of the short stub in FIG. 1 in A) lies in that since the stub length can be shortened in the UHF band and simple rectification by D3 is possible, ASK demodulation which is low in output impedance (fast in response) was possible.
C) The reason for using the open stub not using the short stub in FIG. 2 of A) lies in that the stub length is not too much shortened in the microwave band and the electromagnetic wave radiation loss is suppressed to make it possible to maintain the Q value, and since the stub is in a DC and open state, insertion of a capacitor (C2 in case of FIG. 1) required for voltage-doubling rectification became unnecessary and as a result of which the loss due to the inserted capacitor in the microwave band could be eliminated.
D) in FIG. 2, the inductors L1 to L3 are inserted into the RF boosted signal rectifying diode which is not used in FIG. 1.
E) in FIG. 1 in D), the Q value of the stub in the UHF band is sufficiently high and the +j impedance component in stub resonation could compensate for the −j impedance component of the capacitance load.
G) in FIG. 2 in D), the Q value of the stub is not sufficiently high in the microwave band and the +j impedance component in stub resonation cannot thoroughly compensate for the −j impedance component of the capacitance load. Thus, a position where the inductor is to be inserted is set such that the junction capacitance of the rectifier diode is made to resonate with the inserted inductors so as to reduce the capacitance load and to generate a high RF voltage in the rectifier diode element (a position where the rectifier diode element most approaches the stub or a position where it most approaches GND of the RF potential is preferable).
In FIG. 2, a difference between (a) and (b) is that in case of (a), the rectifier diode element most approaches the stub, and in case of (b), the rectifier diode element most approaches GND of the RF potential. In addition, while in case of (b), a common L1 is inserted into D1 and D2, in case of (a), L1 is inserted into D1 and L2 is inserted into D2. In this case, since the connection conditions of D1 and D2 are different from each other, a higher boost ratio can be obtained by the inductor insertion method of (a).
In (c) of FIG. 2, the stub resonance boosted RF signal is voltage-doubled rectified in D1 and D2 to charge C2 and a doubled rectified voltage of the stub resonance boosted RP signal is added by using D3 and D4 with the direct current potential of C2 used as a reference to charge C4, thereby obtaining a quadrupled rectified voltage. At that time, C3 is inserted for the purpose of separating the DC potential of the stub from the DC potential of the common terminal of D3 and D4.
The ASK demodulation method by U1 in FIG. 1 is the same as the ASK demodulation method by U1 in FIG. 2, and after the received signal has been stub resonance boosted rectified detected, determination as to 0 and 1 which are received data codes is performed by a comparator operation using ½ of the amplitude peak potential as a threshold value.
FIG. 3 is an example of a conventional semi-passive wireless tag device and a circuit diagram described in FIG. 4 of Patent Document 2.
In this figure, the capacitor C1 connected to an antenna element 22 resonates with a λg/4 short stub 23 and boosts the amplitude of the antenna received RF signal by about 10 times. It is made such that while the boosted RF signal is connected to an anode of D3 via C2 and is detected, a DC bias current flows into the anode of D3 via R3. This DC bias current aims to generate an input offset voltage in order to improve the detection sensitivity of D3 to a weak RF received signal and to generate an input offset voltage in order to operate a comparator U1 by a single power source. In addition, it is made such that DC bias current which is slightly smaller than that into the D3 flows into D4 to which the RF signal is not applied via R4. Therefore, in this circuit, in the absence of the RF input signal, VD3>Vd4, and in the presence of the RF input signal, VD<D4 so as to obtain ASK demodulated data from the comparator U1. However, all the embodiments described in Patent Document 2 including FIG. 3 involve the 2.45 GHz band and it was found that the receiving sensitivity required in the wireless tag cannot be obtained in the microwave band.
For example, while a minimum receiving ASK demodulation sensitivity of the circuit in FIG. 3 in the 2.45 GHz band was −45 dBm, it was about −30 dBm in the 5-GHz band. It is thought that the cause for great degradation of the receiving sensitivity of the circuit in FIG. 3 in the microwave band as described above lies in that as in the case of the circuit in FIG. 1, (1) the Q value of the short stub itself has been reduced, (2) the junction capacitance of D3 which is connected via C2 has worked as the load on the stub resonant circuit and has further reduced the Q value of the resonant circuit, and (3) C2 which has been inserted in order to separate the stub which is at the DC and GND potential from the biased potential of D3 has caused not negligible loss to generate in the microwave hand.
FIG. 4 is an ASK demodulation circuit using the microwave band booster rectifier circuit of the present invention, which is mainly used in a semi-passive type wireless tag device. Values and type names of respective elements used in this figure are ones all used in the experiment in the 5 GHz band and an ASK demodulation minimum input sensitivity was 50Ω, −48 dBm with input into RFin. In addition, in the figure, RFin is an input terminal of the received RF signal from the wireless tag antenna, RxData is an ASK demodulation output terminal of that signal, RxEnable is a control input for bringing U1 to U3 into active states in reception standby.
In the circuit in FIG. 4, C1 connected to RFin resonates with the λg/2 open stub to boost the amplitude of the input RF signal by about 10 times and this is voltage-doubled rectified by D1 and D2 to charge C2. On the other hand, a DC bias voltage is obtained at C3 by voltage division of R1 and R2 and it supplies forward bias currents to D1 to D4. Here, the reason why the D1 and D2 are forward biased lies in that a DC offset voltage is needed for the input signal for the purpose of preventing degradation of detection sensitivity to the weak RF signal and in order to operate U2 by a single power source.
Although the bias currents which flow into D1 and D3 and into D3 and D4 when no input signal is present in RFin are respectively determined by R3 and R6+R7, they are made such that the former becomes slightly larger due to a difference in resistance value and the voltage to be charged into C4 becomes slightly higher than the voltage to be charged into C2. In addition, the respectively charged voltages are connected to an operational amplifier U2 via R4 and to an operational amplifier U3 via R5. Feedback resistances of the operational amplifiers U2 and U3 use R11 in common, and the U2 side uses R9 and D6 and the U3 side uses R10 to control gains.
The reason for use of D6 in a feedback circuit of U2 lies in that AGC control is performed so as not to bring U2 into a saturated state even in a case where a comparatively large signal has been input into RFin. In addition, C4 is charged with output from U2 via R8 and D5 so as to raise the input voltage on the U3 side because there exist such effects that the influence of noise is suppressed and the ASK demodulation response performance to detection response of first-order lag is increased by making it follow a threshold value (an output of U3) of the ASK demodulation comparator U1 to which an output of U2 is connected in accordance with the average amplitude of the input signal into RFin.
In the following, differences between FIG. 3 of the conventional system and FIG. 4 of the present invention will be described.
A) in FIG. 3, the short stub is used for resonant boosting of the RF signal, and in FIG. 4, the open stub is used.
B) The reason for use of the short stub in FIG. 3 of A) lay in that the stub length can be shortened in the UHF band.
C) The reason for use of the open stub in FIG. 4 in the A) not using the short stub lies in that the stub length is not shortened too much in the microwave band, the electromagnetic wave radiation loss is suppressed and therefore the Q value can be maintained, and since the stub is in the DC and open state, insertion of the capacitor (C2 in the case in FIG. 3) needed for bias application is not needed, and as a result of which the loss due to the inserted capacitor in the microwave band could be eliminated.
D) in FIG. 4, the inductors L1 and L2 are inserted into the RF boosted signal rectification diode which is not used in FIG. 3.
E) in FIG. 3 in the D), the Q value of the stub is sufficiently high in the UHF band and the +j impedance component in stub resonance could compensate for the −j impedance component of the capacitance load.
F) in FIG. 4 in the D), the Q value of the stub is not sufficiently high in the microwave band, and the +j impedance component in stub resonance cannot thoroughly compensate for the −j impedance component of the capacitance load. Therefore, the junction capacitance of the rectifier diode and the inserted inductor are made to resonate with each other to reduce the capacitance load and the insertion position of the inductor is set so as to generate a high RF voltage in the rectifier diode element (the position where the rectifier diode element most approaches the stub or the position where it most approaches GND of the RF potential is preferable).
G) in FIG. 3, since a simple system of detecting the RF signal by D3 is used, the output impedance of a detection output is comparatively low and the time constant of first-order lag is small, comparatively high-speed demodulation of the ASK signal was possible even in the absence of a follower circuit for the comparator threshold value to the RF input signal amplitude average value.
H) In FIG. 4, although there was the effect of improving the detection sensitivity by utilizing voltage-doubling rectification detection by D1 and D2, the output impedance of the detection output was increased and the time constant of first-order lag was increased. Therefore, addition of the follower circuit for the comparator threshold value to the RF input signal amplitude average value is needed in case of performing demodulation of a comparatively high-speed ASK signal.